Electron discharge devices employing photo-conductive target electrodes



Sept. 22, 1959 'H. G. LUBSZYNSKI ELECTRON DISCHARGE DEVICES EMPLOYINGPHOTO-CONDUCTIVE TARGET ELECTRODES Filed Feb 6, 1956 oLAss awn/0044'ELECTR CALLY CONOUCT/VE F LM COAT/A/G 0F MATERIAL /A/ERT 7'0 PIIJTDCDNOl/CT/VE MATER/AL LCTRICA LL y CONDUCT/VF FILM POIVGY AYER 0F 7/107'0'NDVCT/Vl MATERIAL 50L!) LAYER F 4 ,9070 Cd/VOVL'f/VE MATLRIAL ILac/entai Gjubszymk/ United States Patent ELECTRON DISCHARGE DEVICESEIVIPLOYING PHOTO-CONDUCTIVE TARGET ELECTRODES Hans Gerhard Lubszynski,Lawrence, England, assignor to Electric & Musical Industries Limited,Hayes, Middlesex, England, a British company Application February 6,1956, Serial No. 563,801

Claims priority, application Great Britain February 15, 1955 6 Claims.(Cl. 31365) The present invention relates to photo-sensitive devicesemploying a layer of photo-conductive material and to an improved methodof forming such layers.

A very important factor of photo-conductive devices such as televisionpick-up tubes is the photo-conductive lag of the photo-conductive layerand such lag depends to a large extent on the constitution and thethickness of this layer.

It is often desirable to include in such devices an electrode formed ofa metal which is chosen for its mechanical and electrical properties,such as one of the alloys known under the trade names of Eureka orConstantan said alloys comprising approximately 60 percent copper andapproximately 40 percent nickel. These alloys will re-act withphoto-conductive materials such as antimony trisulphide so that whenemployed in such devices it is difficult to estimate the amount ofphoto-conductive material which will be required to repeatedly producetubes having layers of the same constitution and thickness.

It is therefore an object of the present invention to reduce interactionbetween an electrode of a photo-conductive tube and the photo-conductivematerial employed therein whereby the constancy of the thickness of thephoto-conductive layer of the target electrode of the tube can beimproved.

According to the invention there is provided a photosensitive devicehaving a target comprising a continuous layer of photo-conductivematerial, a tubular electrode composed of a metal which reacts with thephoto-conductive material of said layer mounted adjacent to said layer,and a coating of a material which is substantially inert to saidphoto-conductive material provided on at least a portion of the innerwall of said electrode.

In order that the invention may be clearly understood and readilycarried into effect, an embodiment of same will now be more fullydescribed with reference to the accompanying drawing which shows inFigure 1 the invention applied to the formation of a photo-conductivelayer for a television pick-up tube, Figure 2 being a portion of thetarget of the tube of Figure 1 on a much larger scale. The particularpick-up tube shown is one which is suitable for operation with a lowvelocity scanning beam to restore the scanned surface of thephoto-conductive layer to a potential at or near that of the cathode ofthe tube. For the purpose of illustration the tube shown in the drawingis one having a photo-sensitive layer formed as a double layer ofphoto-conductive material, the first layer being of a porous or spongynature and the second layer being of a solid nature. The method offorming devices having such layers is more fully described in thespecification of co-pending application Serial No. 563,800 and for thisreason only a brief mention of it will be made in the presentapplication.

Referring to the drawing, the inner surface of a planar glass window 1closes one end of a glass envelope 2 and is provided with a targetelectrode comprising a translucent electrically conducting signalelectrode 3 having a first layer 4 of a photo-conducting materialdeposited in porous form and thereover a second layer 5 of aphotoconductive material deposited in solid form. At the opposite end ofthe envelope 2 there is provided an electron gun with its thermioniccathode 6 facing the target and between the gun and target a tubularelectrode 7 serving in operation of the tube as an anode having its endadjacent to the target covered with a mesh 8 preferably formed of silverand which in operation of the tube serves as an ion trap. It will berecognized that the device shown in Figure 1 is of the Vidicon type andas it usual with such a device the envelope 1 when the device is set upfor operation will be surrounded with a suitable coil forelectro-magnetically focussing the electron beam from the gun into awell defined beam and further coils for scanning said beam over thesurface of the layer 5 of the target. Alignment coils may also beprovided in known manner for suitably directing the electron beamtowards the target.

.The materials available for the construction of the tubular anode 7 soas to provide a strong mechanical structure having a sufliciently highresistivity so that it will not unduly screen the penetration of thescanning fields for the electron beam are limited and the most suitablematerials known for this purpose are alloys comprising approximately 60percent copper with approximately 40 percent nickel such as those knownas Constantan and Eureka wherein the metal employed is of 0.002 inchthickness, although other alloys such as stainless steel or Nichrome areused.

In constructing the tube the above-mentioned gun and electrodes 7, 8 areassembled in the envelope 2 and a tantalum boat 9 containing a charge ofphoto-conductive material 10 such as antimony trisulphide is introducedthrough a side tube 11 so that it is at the centre of the envelope 2 andfacing the mesh 8 and target area. The envelope 2 is filled with a gassuch as xenon through pump stem 12 at a pressure of approximately 0.4mm. Hg and with the end of the side tube 11 remote from the envelope 1closed the boat 9 is heated so that the whole of the charge isevaporated. The antimony trisulphide is thus deposited through the mesh8 to form the spongy base layer 4 over the target area. During thisevaporation only part of the evaporated charge deposits on the targetarea, part of it being deposited on the bars of the mesh 8 and theremainder on the inside surface of the tubular electrode 7 over a bandabout 1 inch wide approximately symmetrical to the plane of theevaporator boat 9 and normal to the axis of the tube. The boat 9 ismoved into the side tube 11 and this tube 11 is sealed off close to thewall of the envelope 2. The envelope 2 is evacuated via pump stem 12after which the electrode 7 and mesh 8 are inductively heated so as tore-evaporate the material from the electrode 7 and mesh 8 to deposit itas a solid layer 5 over the spongy base layer 4. During this latterevaporation the target surface is maintained cool by applying water tothe outer surface of the window portion 1 of the envelope 2.

The thickness of the spongy layer 4 is determined by the amount of theoriginal charge of material 10 in the evaporator boat 9, the shadowratio of the mesh 8 and the gas pressure and this can be reasonablycalculated in order to obtain a desired low capacitance for such layer.

The thickness and construction of the solid layer 5 will thus depend onthe amount of material which is deposited on the bars of the mesh 8 andthe inner surface of the electrode 7, but despite carefully controlledprocessing it is found that the thickness of this second solid layer 5will vary from tube to tube with consequent variations in thesensitivity of the tubes.

This is attributed to the fact that during heating of the mesh 8 andelectrode 7 the photo-conductive material reacts to some extent with themetal from which the electrode 7 isformed whereby only a proportion ofthe photo-conductive materialisevaporated to form the solid layer 5 onthe target.

It has been found that this reaction can be substantially prevented ifthe surface of the electrode on to .which photorconductive material isdepositedis first provided with a coating of a material ,13 which is.inert to the .photorconductive material employed in the tube. Tubesmadesin this mannerrhave been found to have .a uniform layerthieknessand uniform sensitivities.

.In atubeinwhich the photo-conductive material '10 is antimonytrisulphide, sufficient protection is afforded by coating, such as byplating, the internal surface of the electrode 7 with rhodium. Othermetals such as gold, platinum, oriridium which are known to besubstantially inertto photo-'conductir e materials such as antimonytrisulp'hide or selenium are also satisfactory for this purpose.

.The dimensionsfor an electrode 7 which is typical for a tube of theabove construction is 0.8 inch diameter and three inchesinlength and itis found that adequate protectionzis alfordedif the inner wall of thiscylinder is plated with one of the above protecting metals over a lengthof 2 /2 inches from the end thereof .to which is secured the mesh 8.

Although the invention has been described with reference to theproduction of tubes having a double layer .4, 5 of antimony trisulphideas the photo-conductive material ,for the target electrode it is alsoapplicable to tubes employing only a single layer of antimonytrisulphide or other photo-conductive materials such as selenium,cadmium sulphide, zinc sulphide, zinc selenide.

Further, although-the invention has beendescribed as applied to theconstruction of pick-up tubes suitable for operationwithlow velocityscanning, it is applicable to pick-up tubes with .high .velocityscanning and generally to devices having a photo-sensitive layerof-photo-conductivematerial in which an electrode is provided whichrequires to be protected against the action of thephotoconductivematerial employed in theconstructionof the device.

What I claim is:

l. A light sensitivedevice having a target comprising a continuous layerof photo-conductive material, a tubular electrode composed of a metalwhich reacts with the photo-conductive material of said layer mountedadjacent to said layer, and a coating of a material which issubstantially inert .to said photo-conductive material provided on atleast a portion of the inner wall ofsaid electrode.

2. A light sensitive device having a target comprising a continuouslayer of a photoconductive material, a tubularelectrode composed of ametal which reacts with the photo-conductive material of said targetmounted .adjacentto saidtargetand acoating on at least a portion of theinner Wall of said electrode of a metal selected from the group rhodium,gold, platinum or iridium.

3. A light sensitive deyj ce having a target comprising a porous layerof a photo'conductive material and coverin sai i c ca laye eeen nuusselid layer o nhot conduetivermateri l, .-..a:tu.b111 r electrodecomposed of a metal whichreacts with the photo conductive material o s dtar steamed adjac n said t r an faci said layer, and ac'oating ofa-rnaterial which is substantially inert to the photo conductivematerial of said target-providedon atrleasta portiontof the inner wallof said electrode.

4. A lightsensiti-vegdeyice having -a target comprising a continuouslayer of a' photo-conductive material, a tubular electrode composd ofanalloy of copper and nickel mounted adjacent to said layer and a coatingof a material which is substantially inert to said photoconductive-material on at: least aportion of the innerwall of saidelectrode.

5. A light sensitive device havinga-target comprising a layerofiantimony-tri sulphide, a tubular electrode composcdota metal whichreacts with antimonytri-sulphide mounted adjacentsaid-target andacoatingof a material which is inert to antimony tri-sulphide on atleast a portion of the :inner wall of said electrode.

:6. A light sensitive device having within its envelope a targetcomprising a porous layerof a photo-conductive material and coveringsaid porous layer a continuous solid :layer of a photo conductivematerial, a tubular electrode .composed of a metal which reacts with thephotoconductive material of said target, said electrode being mounted insaid envelope adjacent said target and supporting aimetal meshextendingtransversely of said tubularteleetrodeatthe end thereof whichis nearer said target, and a coatingof amaterialwhich is substantiallyinert-to ,the-photosconductive material of said target on at least a.portionof the inner wall of said electrode.

Reereneesited in the file of this patent )UNLTED STATES PATENTS

